Implantable medical devices may be used to deliver therapeutic electrical stimulation to patients to treat a variety of symptoms or conditions, such as chronic pain, tremor, Parkinson's disease, epilepsy, incontinence, or gastroparesis. To treat such symptoms or conditions, an implantable medical device may deliver stimulation via electrical leads that include electrodes located proximate to the spinal cord, pelvic nerves, or stomach, or within the brain of a patient. In general, implantable medical devices deliver stimulation in the form of electrical pulses. Implantable medical devices coupled to electrode-carrying leads may additionally or alternatively be used to sense electrical activity within a patient.
An electrical lead may carry multiple electrodes, and each electrode may be coupled to a respective insulated conductor within the lead. An electrode, associated conductor, and tissue proximate to the electrode may form an “electrical path.” The impedance of the various electrical paths provided by a lead may vary over the life of an implantable medical device due to, for example, material degradation or tissue growth proximate to the electrode. Further, a lead may develop a short between two or more conductors when insulation fails, or a conductor may fracture due to bending or other stresses placed on the lead by patient movement or manipulation.
Changes in lead impedance impair the ability of an implantable medical device to effectively sense electrical activity and/or deliver stimulation. Consequently, it may be desired to identify such changes in order to take corrective action, such as implantation of a new lead, or selection of different electrodes for sensing or delivery of stimulation. Traditionally, clinicians have used a programming device during an office visit to manually direct an implantable medical device to perform a lead integrity or functionality test. Manual lead functionality testing may include manually defining a plurality electrode combinations and, for each combination, directing the implantable medical device to measure the impedance presented by electrical paths the combination, or another electrical parameter for the combination, such as the current flowing through the electrical paths. The impedances, currents, or other electrical parameters of the various combinations may allow the clinician to identify changes in impedance or failures of the electrical path associated with a particular electrode.
Manual testing may be desired to confirm and maintain device efficacy, but is also very tedious. Because of the large number of possible electrode combinations that may be available on the one or more leads coupled to an implantable medical device, testing can take several minutes. During this time, therapeutic stimulation is generally not available, which can result in discomfort for or danger to the patient because symptoms are not suppressed.
Furthermore, significant changes in impedance, or other lead functionality issues, may occur between clinic visits, and may occur gradually over time. In some cases, such as where an implantable medical device is used for sensing, or to deliver deep brain stimulation, which are generally not perceivable by the patient, the patient may not detect a degradation of lead functionality. In such cases, the sensing or therapy may be inadequate for a significant period of time, e.g., until the next regularly-scheduled clinic visit, which may pose risks for the patient.
Also, a conductor short or fracture may be intermittent, and more likely to manifest during periods when the patient is changing posture, within a particular posture, or otherwise active. In such cases, a clinician may not be able to detect a conductor problem with a manual lead functionality test performed during an office visit. The existence of undiscovered conductor problems may limit the effectiveness of therapy and sensing, as discussed above.